Commodore VIC-20 User Manual page 94

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The VIC 20 User Guide
technique could cause confusion for other programmers trying to read your
code. Even you might later decide you had mistakenly left out the subscript,
and try to fix the "error."
BASIC Commands
In Chapter 2 we described a number of commands that can be entered
at the keyboard in order to control VIC computer operations. RUN is one
such command. Commands can all be executed as BASIC statements.
You are unlikely to execute commands out of BASIC statements when
you first start writing programs. However, when you start writing very large
programs you may run out of memory space. Then you must break a
program into a number of smaller modules and execute them one at a time.
Reserved Words
All of the character combinations that define a BASIC statement's
operations, and all functions, are called reserved words. Table 3-4 lists the
VIC BASIC reserved words. You will encounter many of these reserved
words in this chapter, but others are not described until later chapters.
When executing BASIC programs, the VIC computer scans every
BASIC statement, seeking out any character combinations that make up
reserved words. The only exception is text strings enclosed in quotes. This
can cause trouble if a reserved word is embedded anywhere within a variable
name. The VIC computer cannot identify a variable name by its location in
a BASIC statement. Therefore, you should be very careful to keep reserved
words out of your variable names. This is particularly important with the
short reserved words that can easily slip into a variable name.
Some rese.rved words are shown in Table 3-4 with an asterisk. These
words are added to BASIC by certain plug-in cartridge programs supplied
by Commodore. They can be used in standard VIC BASIC programs
without causing an error. Nevertheless, it is a good idea not to use these
reserved words in any VIC BASIC program. You may at some point want to
upgrade your VIC with a new cartridge or change a program so that it runs
on another VIC using one.